Interconnection of control units, sensors, and actuator devices via a communication system has increased markedly in recent years in the construction of modern motor vehicles and machines, in particular in the area of machine tools and in automation. Synergy effects can be achieved by distributing functions over a plurality of control units. Such configurations are referred to as distributed systems. Communication between different stations is taking place increasingly via a bus. The communication traffic on the bus, access and reception mechanisms, and error handling are regulated by a protocol. Bus-specific elements of control units, sensors and actuator devices, referred to below as users, in combination with the communication network, are referred to below as a bus system.
An established protocol in the motor vehicle field is the CAN (Controller Area Network). This is an event-driven protocol; that is, protocol activities such as transmission of a message are initiated by events that originate outside of the communication system. Unambiguous access to the communication system or bus system is resolved through priority-based bit arbitration. This requires assignment of a unique priority to each message. The CAN protocol is very flexible; additional nodes and messages can be added readily, as long as there are still free priorities (message identifiers) available. The collection of all of the messages to be transmitted on the network including priorities and their sender nodes, and possibly reception nodes, are stored in a list known as the communication matrix.
An alternative to event-driven, spontaneous communication is the purely time-controlled approach. All communication activities on the bus are strictly periodic. Protocol activities such as transmission of a message are triggered only after the elapse of a time applicable to the entire bus system. Access to the medium is based on the allocation of time ranges in which a transmitter has exclusive transmission rights. The protocol is comparatively inflexible; adding new nodes is only possible when the corresponding time ranges have already been released beforehand. This circumstance forces the order of messages to be set already before start-up. A timetable is therefore created, which must fulfill the requirements of the messages in regard to repetition rate, redundancy, deadlines etc. This is known as the “bus schedule.” The positioning of the messages within the transmission periods must be synchronized with the applications that produce the contents of the messages, in order to keep the latencies between application and moment of transmission to a minimum. Failure to perform this synchronization would destroy the advantage of time-controlled transmission (minimal latency jitter when transmitting the message over the bus). Thus, the demands on the planning tools are high.
The approach to time controlled CAN shown in German Published Patent Applications Nos. 100 00 302, 100 00 303, 100 00 304 and 100 00 305, the so-called TTCAN (Time Triggered Controller Area Network), satisfies the requirements for time-controlled communication sketched out above, as well as the demands for a certain measure of flexibility. TTCAN meets these demands by structuring the communication round (basic cycle) in exclusive time windows for periodic messages of specific communication participants, and in arbitration time windows for spontaneous messages of multiple communication participants.
The bus system may be implemented as a TTCAN, although this is not to be understood as restrictive in regard to the present invention. Rather, the object of the invention explained later may also be employed for additional comparable bus systems.
In the context of a TTCAN network, German Published Patent Application No. 100 00 305 shows a method and a device for exchanging data in messages between at least two bus users which are connected by a bus system, where the messages containing the data are transmitted over the bus system by the users and each message contains an identifier that characterizes the contained data, with each user deciding on the basis of the identifier whether it will receive the message. At the same time, the messages are time-controlled by a first user by having the first user transmit a reference message repeatedly over the bus in at least one specified time interval, and the time interval is subdivided into time windows of specifiable length, with the messages being transmitted in the time windows.